Hi JT, hope this helps.Per Assembly Bill No. 68 (AB-68):"If there is an existing primary dwelling, the total floor area of an attached accessory dwelling unit shall not exceed 50 percent of the existing primary dwelling." 

So I’ve found a house that I would have to do some work to, but at the end would be able to rent it out by the room. This property also has a 15x30 ADU. I’d like to know if it’s possible to turn this into an additiona...

1) It is not yet clear but it seems that you will be grandfathered in at the time when the changes occur in regards to occupancy changes.2) Per AB-68:"(A) One accessory dwelling unit and one junior accessory dwelling unit per lot with a proposed or existing single-family dwelling if all of the following apply:(i) The accessory dwelling unit or junior accessory dwelling unit is within the proposed space of a single-family dwelling or existing space of a single-family dwelling or accessory structure and may include an expansion of not more than 150 square feet beyond the same physical dimensions as the existing accessory structure.

Per AB-68: "(A) One accessory dwelling unit and one junior accessory dwelling unit per lot with a proposed or existing single-family dwellingR1 with ADU Owner occupancy : Owner occupancy has been removed till 2025 but what happens in 2026 ?

A property generally requires a principle building before an accessory building can be built. 

Paint the walls a fresh white and stage the room with a  modern rug and shower curtain (to conceal the tile) and accessories (towels, a small plant, glass apothecary jars for cotton balls, etc.). 

@Joe Vahedi Every project I've done with recently added units has had some level of difficulty with appraisals - I have had to go out of my way to prove that the additional unit is permitted and compliant with zoning in order to qualify it with the underwriter and the income from it.Here's Fannie Mae's guidance on it: https://selling-guide.fanniemae.com/Selling-Guide/Origination-thru-Closing/Subpart-B4-Underwriting-Property/Chapter-B4-1-Appraisal-Requirements/Section-B4-1-3-Appraisal-Report-Assessment/1736875861/B4-1-3-05-Improvements-Section-of-the-Appraisal-Report-10-24-2016.htm#Accessory.20UnitsLenders and underwriters can speak to the details better than I can, but I would be VERY cautious about assuming a building with an unpermitted unit (even if typical for the neighborhood) would appraise with that unit included. 

I'm seeking advice on how difficult it may be to convert a SFH with an ADU (accessory dwelling unit) into a legal Duplex.

"(3) (A) A local agency, special district, or water corporation shall not impose any impact fee upon the development of an accessory dwelling unit less than 750 square feet.

Not sure what you'd call this, but when we bought our sfh back in 2016 California had just passed the Accessory Dwelling Unit ADU law.